elysia

Yep, I sent one in another comment https://github.com/elysiajs/elysia/blob/94abb3c95e53e2a77078... This is not the easiest to follow code, but it's very similar to what you'd find in any fluent web router, the idea is that you have say an App class, which has a Routes generic, then on every route you add you compose the return types by returning this as App<Routes & NewRoute>, the thing is in the most simple cases you can probably do this cast directly and it will be fine, but as you add more features (things like extensibility with plugins, ability to merge to other app routes, etc..) you might eventually run into limitations of the type system that require a escape hatch like "as unknown" or "as any" It's not the only case in which you might use it, but I think Elysia is a great example as it does some really interesting things with the

If you disagree, you're welcome to prove me wrong! To give you an example from a popular open source ts-heavy project: https://github.com/elysiajs/elysia/blob/94abb3c95e53e2a77078... The `return this as any` there, which effectively casts it to the same type this had, but with the added get route is perfectly safe, it works, and will never be a problem by itself.